1. Field of the Invention
This invention relates to the field of bandgap voltage reference circuits, and particularly to circuits and methods that compensate for the bandgap curvature term in the outputs of such circuits.
2. Description of the Related Art
Voltage reference circuits generate one or more reference voltages that are ideally stabilized over process, supply voltage, and temperature variations. Reference circuits which create an output based on the bandgap voltage of silicon largely achieve these ideals, and are one of the most popular types of voltage reference circuit.
The output of a conventional bandgap reference circuit is about 1.25 volts. This typically requires that the supply voltage for the reference circuit be no lower than 1.25 volts. However, there is an ever-increasing demand for low power and low voltage operation, which may make this limitation unacceptable.
A number of bandgap references have been proposed which overcome this supply voltage limitation. One such circuit is described in “A CMOS Bandgap Reference Circuit with Sub-1-V Operation”, Banba et al., JSSC Vol. 34, No. 5, May 1999, pp 670-674. This reference circuit provides a temperature compensated reference voltage with a supply voltage of less than 1 volt. However, the output of a basic bandgap reference circuit compensates for the temperature dependencies of the output voltage only to a first order. One reason for this is that the base-emitter voltage (Vbe) of a bipolar transistor does not change linearly with temperature. This nonlinearity results in a “bandgap curvature” error in the output voltage which varies over temperature. The circuit described in Banba does not address this error, and as such, its reference voltage output may not be adequate for some applications.
Various approaches to compensate for the nonlinearity of Vbe have been proposed. One such approach is described in “Curvature-Compensated BiCMOS Bandgap with 1-V Supply Voltage”, Malcovati et al., JSSC Vol. 36, No 7, May 1999, pp 1076-1081. Here, additional transistors and resistors are added to the reference circuit to provide curvature compensation. However, the additional components have relatively large values and require relatively large areas, adding cost and complexity to the design.